1. Field of the Invention
This invention relates to an electromagnetic flowmeter for measuring the velocity or the amount of flow of various fluids, and more particularly to an electromagnetic flowmeter with an improved electrode mounting structure suited for a ceramic measuring tube or a plastic measuring tube.
2. Description of the Related Art
To meet the recent demands for better insulating properties, higher chemical resistance, and high-temperature use, electromagnetic flowmeters with a ceramic measuring tube for conductive fluid have been developed. Use of such a ceramic measuring tube, however, requires particular attention to the sealing of its electrode.
Some electromagnetic flowmeters with an improved mounting structure have been proposed with a view to fulfilling the foregoing demands.
For one flowmeter, as shown in FIG. 8, electrode mounting holes 2 of a specified diameter are made in the ceramic measuring tube 1 on the center line, each on opposite sides of the tube. Externally fitted into each electrode mounting hole 2 is an insulating bush 3 whose cross section is shaped like a "T". Over the inside surface of the ceramic measuring tube 1, a Teflon lining 4 of a specified thickness is formed. An electrode 5 whose cross section takes the form of an anchor (hereinafter, referred to as the anchor-shaped electrode) is inserted in the bush 3, penetrating the Teflon lining 4 from inside the tube 1. The anchor-shaped electrode 5 is installed so that its top end may bite into the Teflon lining 4 with its bottom end locked in position with a screw via an elastic member 6. The elastic member 6 is normally pulling the electrode 5 toward the inside of the measuring tube 2. This ensures that the electrode 5 is tightly sealed with the biting of the electrode top end into the Teflon lining 4 and the bush 3.
For another flowmeter, as shown in FIG. 9, there are electrode mounting holes 2 of a specified diameter in the ceramic measuring tube 1 on the center line, each on opposite sides of the tube. Concave portions are formed around the periphery of each electrode mounting hole 2 inside the ceramic measuring tube 1. The anchor-shaped electrode 5 is inserted in each electrode mounting hole 2 from inside the measuring tube 1 via an O ring packing 8. The bottom end of the electrode 5 is locked in place with a screw via a spring 9. Thus, this electrode mounting arrangement is such that the measuring tube 1 is kept sealed only by the thickness of the tube 1 and the O ring packing 8.
of the aforementioned electromagnetic flowmeters, the FIG. 8 flowmeter permits the biting portion of the anchor-shaped electrode 5 and the corresponding bitten portion of the Teflon lining 4 to abrade, resulting in a degraded sealing. A fluid containing sand or gravel also wears out the electrode 5, raising a similar problem of losing sealing capabilities.
In the case of an electromagnetic flowmeter with an O ring packing 8, from the viewpoint of installation of the O ring packing 8 and better sealing properties, it is necessary to use an expensive, thick-walled ceramic measuring tube 1 or a reinforced plastic tube (such as, a plastic measuring tube reinforced with boron, carbon, or glass fiber). Since ceramic is brittle, there is a possibility that the vibration of the electrode top end and other factors damage the ceramic.
Singer unsigned ceramic creates pores at the surface of the ceramic often leading to a bad surface condition. To remove such a pore scattering layer, it has been a common practice to grind or chemically polish that portion thoroughly. Ceramic, however, has the disadvantage of being not only brittle but also easily broken at the portions where it is in contact with the electrode. In addition, grinding can create fine hair cracks, which might develop into breakage due to expansion when a large fluid pressure is applied in practical use.